# std::function_ref::function_ref

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< cpp‎ | utility‎ | functional‎ | function ref

template< class F > function_ref( F* f ) noexcept; | (1) | (since C++26)
template< class F > function_ref( F&& f ) noexcept; | (2) | (since C++26)
template< auto f > function_ref( std::nontype_t<f> ) noexcept; | (3) | (since C++26)
template< auto f, class U > function_ref( std::nontype_t<f>, U&& obj ) noexcept; | (4) | (since C++26)
template< auto f, class T > function_ref( std::nontype_t<f>, /*cv*/ T* obj ) noexcept; | (5) | (since C++26)
function_ref( const function_ref& other ) = default; | (6) | (since C++26)

Creates a new `std::function_ref`.

1) Initializes `bound-entity` with f, and `thunk-ptr` with the address of a function `thunk`. The behavior is undefined if f is a null pointer.

- This overload participates in overload resolution only if both std::is_function_v<F> and /*is-invocable-using*/<F> are true.

2) Initializes `bound-entity` with std::addressof(f), and `thunk-ptr` with the address of a function `thunk`.

- Let T be std::remove_reference_t<F>. This overload participates in overload resolution only if :
  - std::remove_cvref_t<F> is not the same type as `function_ref`,
  - std::is_member_pointer_v<T> is false, and
  - /*is-invocable-using*/</*cv*/ T&> is true.

3) Initializes `bound-entity` with a pointer to an unspecified object or null pointer value, and `thunk-ptr` with the address of a function `thunk`.

- Let F be decltype(f). This overload participates in overload resolution only if /*is-invocable-using*/<F> is true.
- The program is ill-formed if f != nullptr is false when std::is_pointer_v<F> || std::is_member_pointer_v<F> is true.

4) Initializes `bound-entity` with std::addressof(obj), and `thunk-ptr` with the address of a function `thunk`.

- Let T be std::remove_reference_t<U> and F be decltype(f). This overload participates in overload resolution only if :
  - std::is_rvalue_reference_v<U&&> is false, and
  - /*is-invocable-using*/<F, /*cv*/ T&> is true.
- The program is ill-formed if f != nullptr is false when std::is_pointer_v<F> || std::is_member_pointer_v<F> is true.

5) Initializes `bound-entity` with obj, and `thunk-ptr` with the address of a function `thunk`. The behavior is undefined if obj is a null pointer when std::is_member_pointer_v<F> is true.

- Let F be decltype(f). This overload participates in overload resolution only if /*is-invocable-using*/<F, /*cv*/ T*> is true.
- The program is ill-formed if f != nullptr is false when std::is_pointer_v<F> || std::is_member_pointer_v<F> is true.

6) Defaulted copy constructor copies the `bound-entity` and `thunk-ptr` of other.

The address of a function `thunk` is used to initialize `thunk-ptr` such that a call to `thunk`(`bound-entity`,`call-args`...) is expression-equivalent to:

Overloads | Expression-equivalence
--- | ---
(1,3) | std::invoke_r<R>(f,`call-args`...)
(2) | std::invoke_r<R>(static_cast<cv T&>(f),`call-args`...)
(4) | std::invoke_r<R>(f, static_cast<cv T&>(obj),`call-args`...)
(5) | std::invoke_r<R>(f, obj,`call-args`...)

/*is-invocable-using*/<T...> is true if and only if:

- std::is_nothrow_invocable_r_v<R, T..., Args...> is true if noex is true, or
- std::is_invocable_r_v<R, T..., Args...> is true

### Parameters

other | - | another `function_ref` to copy from
f | - | a function or a Callable object to wrap
obj | - | an object or pointer to bound

### Example

| This section is incomplete Reason: no example

### See also

(constructor) | constructs a new `std::move_only_function` object (public member function of `std::move_only_function`)

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